The present invention is related to telephone terminal equipment. More specifically, it pertains to circuitry in the key service unit (KSU) of an electronic key telephone system for connecting each key telephone station set in the system to a central office (C.O.) line in the event of a failure of local power at the system installation site.
Known key telephone systems of conventional design have at least one pair of conductors for each line running from the key service unit to each station set, and accomplish switching of any selected line at the station set by means of a key or pushbutton associated with the line. Consequently, the number of conductor pairs increases as the number of central office lines increases, with accompanying increases in cost of materials and labor required to effect an installation. To overcome these problems, electronic key telephone systems have been designed to effect switching at the KSU instead of in the station set, using reduced-pair cable runs to establish voice signal and data signal and power transmission paths between the KSU and each station set. In such systems, the actuation of a key or pushbutton at a station set causes transmission of data from the station set to the KSU, thereby signalling the desired service, e.g., connection to a C.O. line. The KSU might employ time division or space division techniques, or both, with relays and semiconductors being employed to implement space division.
The known way of maintaining telephone service during power failures is to provide additional relays in the KSU for connecting each C.O. line to a predetermined station set. It is the object of the present invention to accomplish the function of establishing the connections of each C.O. line to a predetermined station set in a key telephone system during power failures without using additional relays in the KSU.